1. Field of the Invention
This invention pertains generally to amplifiers, and more particularly to translinear variable-gain amplifiers.
2. Background of the Invention
FIG. 1 illustrates a basic translinear amplifier 10. Translinear refers to the linear dependence of a bipolar transistor's transconductance upon its collector current. Amplifier 10 uses translinear principles to amplify a differential current.
In FIG. 1, amplifier 10 has a first differential transistor pair Q1, Q2 with a current source (having a current magnitude 2ID) regulating their summed tail currents. Amplifier 10 also has a second differential transistor pair Q3, Q4 with a second current source (having a current magnitude 2IN) regulating their summed tail currents. The bases of Q1 and Q4 are connected, and the bases of Q2 and Q3 are connected. The collectors of Q1 and Q2 are coupled to the connected bases through a loop amplifier 12, which may be nothing more than a pair of emitter followers.
The operation of this basic translinear amplifier was analyzed in Barrie Gilbert, “A New Wide-band Amplifier Technique”, IEEE J. Solid-State Circuits, vol. SC-3, no. 4, pp. 353–365 (December 1968). Assuming that the transistors are matched, the common base connections between the inner and outer transistors and the common emitter connections within the pairs force the current ratio between Q1 and Q2 to equal the current ratio between Q3 and Q4. When no differential current is applied to nodes A and B, a current ID passes through each of Q1 and Q2, and a current IN passes through each of Q3 and Q4. When a differential current is applied to Q1 and Q2, it can be defined in terms of a modulation factor x that can vary between −1 and +1, as shown for a differential input current xID. This modulation factor is replicated at the output pair Q3, Q4, modulating the output tail current by the same factor x. The current gain of this cell is simply the ratio of the tail currents, i.e., IN/ID.
Amplifier 10 is useful in variable-gain amplifiers (VGAs) and multipliers. It can be configured as an input VGA (IVGA) by providing for control of the input tail current. In this configuration it is useful, e.g., for conditioning an input signal with a wide dynamic range (such as a received RF signal) before presenting that signal to circuitry that expects signals in a fairly constant range (e.g., automatic gain control applications). The gain varies hyperbolically with changes in the input tail current.
Amplifier 10 can also be configured as an output VGA (OVGA) by providing for control of the output tail current. In this configuration it is useful, e.g., for variable output power/drive applications. Also, since the output current varies linearly with changes in the output tail current, an appropriately designed OVGA can be used as a multiplier.